How do I apply a light/dark mask to a mesh?
It is a bit difficult to understand what you are describing. I would have to assume you are using Cycles, and that you already have a mesh that is UV unwrapped to match the layout of this image. 

If the above is true, you would "apply a light /dark mask"  by using a mix node set to overlay mode. You would plug your underlying surface into the top of the mix node, and plug this image into the bottom of the mix node. Overlay ensures it will lighten or darken as appropriate to your image. You would then increase the factor to get a level of mixture that you desired. 

Hope that helps.